The Building Blocks of Contactors
Contactor Building Blocks
Types of Contactors
How Contactors Operate
Life Expectancy of Contactors
Contactor Video Overview
- Knife-Blade Switches - The first device used to stop and start electric motors was a simple Knife Blade Switch.
This was a lever that would drop a strip of metal onto a Contact to make the electric circuit. In the late 1800's, "throwing the
switch" meant that someone had to stand next to the knife blade switch and level it into the closed position. Engineers discovered
that the contacts quickly wore out because humans could not open and close the switch fast enough to prevent arcing, which corroded
the soft copper switches with pits, making them more susceptible to dirt and moisture.
Knife-Blade Switches were widely used in the early stages of electrical contact (early contactors). In the late 1800's, the phrase "throwing the switch" was coined in reference to the Knife-Blade Switch.More importantly, as motors became larger, the currents to operate them also had to become larger, creating a serious safety concern. It was physically dangerous to handle the switch. Mechanical improvements were made, but with their dangerous operation and short contact life, knife blade switches remained at a design dead-end.
- Manual - The manual controller was the next stop up the evolutionary ladder, offering several important new
- The unit is encased, not exposed
- Double-break contacts are used instead of single-break
- The unit is physically smaller
- The unit is much safer to operate
Double-break contacts open the circuit in two places simultaneously. Dividing the connection over two sets of contacts allows you to work with more current in a smaller space than available with a single-break contact. In addition, the mechanical linkage consistently opens and closes the circuit, sparing the metal from some of the arcing experienced under knife blade switches.
With a manual controller, the operator presses a button or moves a switch that is integral to the electrical equipment being run. In other words, the button or switch is physically attached to the controller itself, and is not operated remotely. When an operator activates a manual controller, the Power Circuit engages, carrying the electricity to the load.
- Magnetic Contactors - Engineers eventually made a breakthrough with the magnetic contactor. A magnetic contactor is operated electromechanically without manual intervention. This means that the contactor can be operated remotely, without the need for putting a person in a potentially dangerous location. Magnetic contactors use a small control current to open and close the circuit.
Mechanically interlocked contactors, like the ones pictured above, are designed for reversing 2 speed, reduced voltage, type starter applications.There are different classifications of contactors used for many different applications. Two of the main classifications for motor control components are the International Electrotechnical Commission (IEC) and the National Electrical Manufacturers' Association (NEMA). The differences between the IEC and NEMA classified components is reflective of their size, speed, functionality and power. IEC and NEMA also rate their components based on different philosophies. North American (NEMA) general purpose machine tool contactors generally emphasizing simplicity of application while definite purpose and European rating (IEC) philosophy emphasizes design for the intended life cycle of the application. Over all, contactors are split between these two classifications. Some examples include:
- AC Mechanically Interlocked - IEC
- DC Mechanically Interlocked - IEC
- AC Reversing - IEC
- AC Non-Reversing - IEC
- AC Reversing - NEMA
- AC Non-Reversing - NEMA
- Movable contacts
- Stationary contacts
Magnetic starters include a contactor as an essential component, while also providing power-cutoff, under-voltage, and overload protection. Electromagnetic contactors are actuated by electromechanical means. They make and break power circuits to such loads as electric furnaces, lights, transformers, capacitors, heaters and, when overload relays or inherent protectors are used, motors. Pushbuttons and selector switches, like the ones on a control panel, are used in hundreds of manufacturing industries. Each button and switch is connected to a contactor for use in making or breaking an electrical circuit remotely. They are used in applications such as elevators, pools, food processing, pumps/compressors, lighting, hoists and cranes, battery chargers, printing presses, vending machines, and agricultural processes.
Electrical arcing is a key contributor to the shelf-life of a contactor. The arc between the contacts creates additional heat which, in time, can damage the contact surfaces.
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